Attempt to add a 0.25 V pk-pk 1 kHz signal into GoPro successful. Oscilliscope gets double the programmed signal amplitude-an effect of impedance mismatch, if I recall.
0.5 V input getting scratchy–think it is poor-quality USB connector…actually, I soldered the connections wrong.
Now it works up to 1 v pk-pk (may go higher). Unfortunately there is automatic gain control. So much for calibrated signals, unless I can feed an oscillating tone into the system or have a calibrated recorder issued nearby…
Wiring diagram for applying external audio
Image courtesy of video provided by DIY Tinker:
[youtube https://www.youtube.com/watch?v=ju2N1S1Pp94]Also, here is a fellow who opened up his GoPro to wire the microphone directly:
[youtube https://www.youtube.com/watch?v=JYhEl9t8zCM&w=560&h=315]and here is how you do the teardown…
Glimpse of what wiring for USB acoustic connector looks like through a microscope. Note relative position of raised tab with wires.